1. Field of the Invention
The present invention relates to structural portal covers and in particular, to an overflow hatch construction for paper pulp bleaching towers.
2. Description of the Prior Art
Paper pulp bleaching towers are vertically elongated tubes or vessels of about 10 feet in diameter and 20 to 40 feet high. Aqueous wood pulp is blended with an appropriate reaction chemical such as chlorine, chlorine dioxide, sodium hydroxide or oxygen and pumped into the bottom of the bleaching tower where the chemical acts upon the lignin or cellulose constituents of the pulp. The process described is a continuous flow process which requires a finite residence or reaction time for the reactive chemical to accomplish the desired objective. This reaction time is gained by the slow linear progression of the pulp along the tower vessel length.
Often, as in the case of chlorine and oxygen, the reaction chemical is blended as a gas and at least some pressure is desired to drive the gas into solution with the aqueous pulp. This pressure at the blending point is provided by the vessel height.
Ideally, by the time the blended pulp reaches the top of the bleaching tower, all the reaction chemical has reactively combined with the pulp and no free gas remains. In practice, however, an excess of chemical is required to assure that all the pulp is reacted. By the time a given increment of blended pulp reaches the vessel top, undissolved or unreacted chemical breaks free of the aqueous mixture to form a gaseous pocket under the vessel top cap. The pulp flows, unpressurized through a withdrawal pipe that penetrates the vessel wall below the top.
In the unusual case, pulp flow must be temporarily interrupted and depending on the interruption sequence, withdrawal may be interrupted prior to influx. This circumstance raises the pulp surface level within the tower vessel into the gas pocket. In the extreme case, the rising pulp fills the gas pocket and must be released through an overflow port at the vessel top. A closure for the overflow port is therefore required to confine the gas pocket under normal conditions but will open to release a pulp overflow and thereafter, close when conditions return to normal.
Such a port closure is normally characterized as a relief hatch which is merely a hinged door across the overflow portal with a flat gasket seal face.
The difficulties with prior art hatch designs arises first from the adhesive characteristic of the pulp fiber which has a strong adhesive affinity to most surfaces and therefore contaminates the hatch seal surface to prevent gas confinement after an overflow.
A second difficulty with prior art bleach tower hatches is the highly reactive nature of the chemicals used. Given sufficient time, most gasket materials deteriorate, in one form or another, from the chemical attack.
It is, therefore, an object of the present invention to provide a pulp bleaching tower hatch that will effectively seal the gas pocket in the tower top from loss to the atmosphere.
Another object of the invention is to provide a hatch seal that resists pulp contamination or obstruction.
Another object of the invention is to provide a hatch seal design that will maintain an effective seal after partial failure from chemical attack.